Carrier signaling system



Nov. 18, 1941. J. E. T. PARKER ET AL 2,263,139

CARRIER SIGNALING SYSTEM I Filed Mrch 5, 1940 2 Sheets-Sheet 1 MRE T2 MRF / N VEN r025 J.E.T.PARKER J. 5. Me CusKER Nov. 18, 1941. J. E. T. PARKER ET AL 2,263,139

CARRIER SIGNALING SYSTEM Filed March 5, 1940 2 Sheets-Sheet 2 .PFZ HPF 63501 390m YB vww.-

/N-VENTOES J. E. T. PARKER M n aw ww q I I I Patented Nov. 18, 1941 CARRIER SIGNALING SYSTEM John Ernest Thomas Parker and Joseph Bernard McCusker, Liverpool, England, :assignors to Automatic Telephone & Electric. Company Limited, London, England, aBritish company Application March 5, 1940, Serial'No.;322,i 402 In Great Britain April 26,-1 939 l "The present invention is concerned with improvements in or relating to telephone systems andmore particularly to systems of the single channel carrier type, that isto say, systems in which a carrier frequency channel is superimposed on a telephone line.

*The main object of theinvention is to provide a simple and improved selective ringing scheme for use on systems of the above described-type.

According to one feature of the invention a'telephone system is provided having a carrier frequency-channel and an audio frequency channel superimposed on the same line in which the signaling for one channel is effected by causing direct current to flow over the line while signaling for the other channel is effected by causing alternating current to flow over the line.

'The invention will be better understood from the following description of one method of carrying it into effect, reference being had to the accompanying drawingscomprising Figs. 1 and 2Iwhichshould' be arranged side by side to form a circuit diagram of terminal equipment as will be provided, at each end of the line in a single channel carrier system according to the invention. f v

Each terminal equipment includes a combined modulator and demodulator unit or so-called mod'em MD which operates in conjunction with a single valve carrier frequency generator OSC, generating anfrequency of the order of 6.5 kilocycles. The arrangements are moreover such that the lower side-band frequency is used for and will be known. as the carrier and audio exv change line respectively, while leads l4 and I5 on the right hand side of the drawing connect up overthe-carrier exchangeline conductors 10, ll, they extend via the low pass filters LPFI having a;pass rangeupto'3000 cycles, and thence via th'e modem'input transformer TI to the modem which is of the fully balanced carriersuppressing-type and employing metal rectifiers MRE-MRH. A carrier frequency of the ordero'f 6-55- k-ilocycles is applied thereto from the oscillator OSC via-the transformer T3, the oscillator comprising vavalve .Vl connected up in an oscillatorycircuit, power to the valve Vl being assumed to'be provided from the 50 volt supply serving the-whole-equipment. although if desired a high voltagemainssupplycouldi be utilised for thepurpose. The oscillatory circuit comprises the primary of the transformer T4, the fixed condenser QEand variable condenser QD. The circuitis connected to the anode of VI over a condenser QB. Thesecondary of the transformer T4 -isinclu-ded in the grid circuit of the valve and'is connected to the grid over resistance YD and to the cathode-over condenser QC shunted by resistance YC. -Aresistance YA is inserted in the filament lead.

The carrier frequency is suppressed inthe modem and the outputfrom' the transformer T2 will inthe main consist of upper and lower sideband frequencies. The secondary windings of this transformer are connected to a low pass filter LPE2 having a passrange up to 6,350 cycles, and hence it will offer a block to the upper sideband frequencies, whilethe lower side-band frequencies will be able to pass through and thence via'the high pass filter HPF, having a pass range At the distant end the received lower sideband will extend viaa high pass filter and low pass filter corresponding to HPF and LPF2 and thence into themodem unit where demodulation with the line to a distant exchange, the line being either an open line or an unloaded cable line not exceeding 10 db attenuation.

Before consideration is given to the question of selective ringing over the carrier channel and audio circuit, the general operation of the system during conversation will be reviewed.

When audio frequency currents are received takes place and produces" the original audio frequencies which thereupon extend to the receiving party.

As regardsuthe handling of speech currents over the carrier channel in the reverse direction, the operations will :be identical except that the modem unit shown-will operate as a demodulator to lower side-bandl' frequencies received over leads l4, l5 and will extend audio frequencies via transformer TLand filter LPFI back on to the carrier exchange line conductors 1.0, II,

Considering the operations when speech-is received over the audio exchange line conductors I2, l3, the speech currents will extend via the low pass filter LPF3, on to the outgoing line conductors l4 and I5, and in this instance the high pass filter HP]? will prevent the feeding back of these frequencies into the modem MD. At the distant end the received audio frequencies are pilotedthrough' a low pass filter corresponding to LPF3 and thence to the receiving subscriber, a high pass filter corresponding to HPF preventing these frequencies from..feeding into the distant modem unit.

Consideration will now be given to'the selective ringing arrangements accordingtothewinvention. When it is desired to ring out over.the line l4,

l from the audio exchange Iine-ringing current at the usual periodicity of 11-25 cycles will; be I applied to the leads I2, I3 and will extend via ringing in the reversedirection the ringing-current received overleads 1,4,. {5 will extend via filter LPF3 back to the leads l2, 13 where it will bring about operation of a signaling device of any of the types already described.

When it is desired to ring out over the carrier "exchange line conductors [0, ll ordinary ringing current will be applied thereto and will bring about the operation of relay R, shownas a standard ,direct current relay connected in circuit with a rectifier bridge comprising rectifiers MRA-MRD, but which might be an alternating current relay. A resistance YE is inserted in one of the leads to the rectifierbridge as shown. Relay R in operating at its armatures T2 and r3 connects up the earthed positivepole of the battery via rectifiers MRL. and MRM through the windings of relay M onto the 1ine;conductors IA, l5. Current then flows through thewindings of relay M and over both line conductors in parallel tonegative battery via a corresponding relay atthe distant end, andas will be understood both relays will operate. The distant relay M in, operating operates a relay C and starts up the distant ringing generator which is maintained in operation by relay C which applies ringing current to the distant.oarriergexchange line. At the originating end relay C is prevented from operating on the operation .of relay M owing to the opening of its circuit by the operated armature rl.

As regards carrier channel signalingfrom the distant end, the simplex'direct current signal received over leads, I4 and I5 operates relay M which willbe connected to negative battery via rectifiers MRK and MRJ andzthisrelay at armature ml earths lead I6 to start up the ringing generator and at armature m2. completes a circuit to relay C which is slightly slow in operating to give immunity from false operation to line clicks and surges. Relay C-in operating maintains an earth on lead l6 over cl, disconnects'the circuit of relay R and connects'up the ringing generator GNRover contactscZ, 03 so as to return a ringing; signal back over the carrier exchange line conductor;l.0,;ll. .i

It will be appreciated that the use of ringing current for the audio circuit in conjunction with direct current for the carrier current channel as far as the line is concerned enables the two circuits to be operated with no interference therebetween either from the signaling or transmission point of view. The ringing current utilised for signaling over the audio circuit is prevented from feeding back into the carrier equipments at each end of the line due to the blocking effect of the 3,900 cycle high pass filters, while the carrier channel D. C. signaling relays M are entirely unresponsive to ringing current, since this current will flow through each of the relays with the windings in series opposition, and furthermore the pair of rectifiers in series with each relay will be in opposing directions of conductivity as far as loop circuit signaling is concerned. The rectifiers also function during conversation over the line either on the audio circuit or carrier channel to oi fer a high impedance at the low value speech voltages so that even though the relays are connected across the lines with their windings in oppositionand are therefore to all intents and purposes non-inductive, the rectifiers in series therewith, will ofi-er such impedance as to reduce transmission losses to negligible proportions. I

The D. C. signaling relays M on the line are normally connected to earth through their batteries and any difference in the battery voltage or earth potential at the two exchanges may cause a circulating current to flow which will bring about false operation, but this is prevented by virtue of the rectifiers connected in circuit therewith, since the pair of rectifiers at one end of the line is connected up in an opposing direction. to that at the other end of the line during such times as the relay R at either end is normal.

It will be noted that when ringing out is taking place from the carrier exchange line II], II, the ringing current which operates relay R is prevented from feeding into the modem MD due to the blocking effect of transformer TI the efliciency of which is very small at the low frequency utilised for ringing purposes.

Facilities are provided for synchronising the carrier frequencies between the local and distant stations, and. this is effected by inserting into the jack JL in each terminating equipment a plug having a telephone instrument connected thereto. As a result, carrier current will be transmitted to line at both ends thereof, and if a beat frequency is heard, the frequency of the carrier frequency generator in the local and/or distant terminating equipment will be varied by means of the variable condenser QD until it disappears, when both carrier sources will be in synchronism with one another. A resistance YB is inserted in one of the leads to the jack. Jack JE simply serves for monitoring purposes on the carrier channel.

In the system described the various filtersprovided in the terminating equipments may be of a very simple type as their only function is to separate the carrier channel from the audio cir cult and vice Versa during signaling and. con versation. I

It will be understood that while the description of the invention has been directedto selective ringing facilities, the invention is not'limited thereto and may be generally applied in telephone systems of the type described Whenever differing to a f distant point.

*We claim V V li In a-telepl;1one system, a line, a path connected to said line including a low pass filter to permit the passage of low frequency alternating current for signaling purposes to both conductors of saidline inaseries circuit as well as speech currents, a second pathnon-conductive to direct currents connected to said'line and including a highpass'filter to permitthe passage of carrier frequency alternating currents to said line, and means for causing direct current to flow over said line for signaling purposes in connection with signals-are-to be extended over a common line saidcarrier frequency currents only.

2. In a'telephone' system, a line, a path connected tosaid line of such a nature as to permit the passage of lowfrequency alternating current for signalingpurposes to both conductors of said line in a series circuit, a second path connected across said lineincluding a relay and two rectifiers connected in opposite senses so as to offer a high impedance at all times to alternating current potentials between the conductors of said line, and means for connecting a pole of a direct current source to a point between said rectifiers to permit the passage of direct current to the two conductors of said line in parallel.

3. In a telephone system, a line, a path connected to said line of such a nature as to permit the passage of low frequency alternating current for signaling purposes to both conductors of said line in a series circuit, a second path across said line including both windings of a double-wound relay and a pair of rectifiers in series therewith between saidwindings and offering a high impedance to alternating current, and means for connecting a pole of a direct current source to a point between said rectifiers to permit the passage of direct current to the two conductors of said line in parallel.

4. In a telephone system, a line, a path connected to said line of such a nature as to permit the passage of low frequency alternating current for signaling purposes to both conductors of said line in a series circuit, a second circuit across saidline including both windings of a double-wound relay, switching means, a pair of rectifiers connected in opposition so as to offer a high impedance to alternating current potentials across said line, a second pair of rectifiers also connected in opposition to each other and in opposite directions to said first pair and arranged to replace said first pair when the switching means are operated, and a direct current source having one pole connected to the intermediate point of said first pair of rectifiers and the other pole connected to the intermediate point of said second pair of rectifiers so as to permit current to flow in opposite directions through the windings of said relay according as to whether said switching means are operated or not.

5. In a telephone system, a line, a path connected to said line including a low pass filter to permit the passage of low frequency alternating current for signaling purposes to said line as Well as speech currents, a second path non-conductive to direct currents connected to said line and including a high pass filter to permit the passage of carrier frequency alternating currents to said line, means for causing direct current to flow over said line for signaling purposes in connection with said carrier frequency currents including a relay and two rectifiers connected in opposite senses so as to oifer a high impedance at all times to alternating current potentials betweenthe conductors of said line, and means for connecting a pole of a directcurrent-source to ajpointbetween said rectifiers to permit the passage of direct current to the two conductors of said line in parallel.

6. Ina telephone system, a line, a path connected to said line including a low pass filter to permit the passage of low frequency alternating current for signaling purposes to said line as well as speech currents, a second path non-conductive to direct currents connected to said line and including. a high pass filter to permit the passage of carrier frequency alternating currents to said line, and means for causing direct current to fiow over said line for signaling purposes in connection with said carrier frequency currents including both windings of a double-wound relay, switching means, a pair of rectifiers connected in opposition so as to offer a high impedance to alternating current potential across said line, a second pair of rectifiers also connected in opposition to each other and in opposite directions to said first pair and arranged to replace said first pair when the switching means are operated, and a direct current source having one pole connected to the intermediate point of said first pair of rectifiers and the other pole connected to the intermediate point of said second pair of rectifiers so as to permit current to flow in opposite directions according to whether said switching means are operated or not.

7. In a'telephone system, a line, a circuit connected to said line of such a nature as to permit the passage of low frequency alternating current for signaling purposes to both conductors of said line in a series circuit, a second circuit connected across said line including a relay and two recti-, fiers connected in opposite senses so as to offer a high impedance at all times to alternating current potentials between'the conductors of said line, switching means for controlling the connection of a pole of a direct current source and said rectifiers to permit the passage of direct current to the two conductors of said line in parallel, and an alternating current relay device for effecting the operation of said switching means.

8. In a telephone system, a line, a circuit connected to said line of such a nature as to permit the passage of low frequency alternating current for signaling purposes to bothconductors of said line in a series circuit, a second circuit across said line including both windings of a double-wound relay and a pair of rectifiers in series therewith between said windings and offering a high impedance to alternating current, means for connecting a pole of a direct current source to a to both conductors of said line in a series circuit, a, second path non-conductive to direct currents and including a high pass filter to permit the passage of carrier frequency alternating currents to said line, a third path including both windings of a double-wound relay and a pair of rectifiers in series therewith between said windings and ofiering a high impedance to alternating current, and a direct current source having one pole connected to a point between said rectifiers to permit the passage of direct current to the two conductors of said line in parallel, said third path serving for the transmission of direct current signaling currents associated with the transmission of carrier frequency currents over said second path.

10. In a telephone system, a line, a path connected to said line of such a nature as to permit the passage of low frequency alternating current for signaling purposes as well as speech currents to both conductors of said line in a series circuit, a second path non-conductive to direct currents and including a speech current receiving circuit, a six termina1 modulator-demodulator unit and a high pass filter to permit the passage of carrier frequency alternating currents modulated by speech currents received by said speech receiving circuit to said line, a third path including both windings of a double circuit relay, switching means, a pair of rectifiers connected in opposition so as to offer a high impedance to alternating current potentials across said line, a second pair of rectifiers also connected in opposition to each other and in opposite directions to said first pair and arranged to replace said first pair when the switching means are operated, a direct current source having one pole connected to the intermediate point of said first pair of rectifiers and the other pole connected to the intermediate point of said second pair of rectifiers, and an alternating current device associated with said speech receiving circuit to respond to 10W frequency alternating signaling currents arranged to control said switching means to permit direct current to flow to the line for signaling purposes associated with the transmission of carrier frequency currents.

JOHN ERNEST THOMAS PARKER. JOSEPH BERNARD McCUSKER. 

